1. Field of the Invention
The present disclosure relates to a method for joining a film onto a substrate. Particularly, the present disclosure relates to a method for joining a solar cell film onto a substrate.
2. Description of Related Art
Patent Literature 1 (U.S. Pat. No. 4,774,194), Patent Literature 2 (United States Pre-Grant patent publication No. 2009/0038678), and Non Patent Literature 1 (M. Konagai et at, Journal Crystal Growth 45(1978)277-280) disclose an epitaxial lift-off method for fabricating a solar cell. The epitaxial lift-off method includes a step of peeling a III-V semiconductor solar cell film with a thickness of several micrometers formed on a GaAs sacrifice substrate from the GaAs sacrifice substrate and a step of joining the III-V semiconductor solar cell film onto another substrate.
More particularly, in the epitaxial lift-off method, a sacrifice layer such as an AlAs layer, a Ga0.3Al0.7As layer, or an AlAs0.98P0.02 layer is formed on the GaAs sacrifice substrate. The III-V semiconductor solar cell film made of, for example, a GaAs film, a GaInP film or an InGaAs film is formed on the sacrifice layer. Subsequently, by using an adhesive resin such as wax, the GaAs solar cell is joined on a for-joining-substrate.
Next, the joined substrates are immersed in hydrofluoric acid or hydrochloric acid. The etching rate of the sacrifice layer by the hydrofluoric acid or the hydrochloric acid is much greater than the etching rate of the solar cell film. Accordingly, only the sacrifice layer is etched and the solar cell film is peeled from the GaAs sacrifice substrate. Optionally, the peeled solar cell film is joined onto another substrate.
The GaAs sacrifice substrate can be reused in the epitaxial lift-off method. Accordingly, an amount of the GaAs sacrifice substrate to be consumed is reduced, and the solar cell may be fabricated at low cost. The solar cell film fabricated by the epitaxial lift-off method may be joined onto a plastic substrate, thereby obtaining a lightweight solar cell. Since lightness is required for components loaded on an artificial satellite, the solar cell joined onto the plastic substrate may be preferable for the artificial satellite.
The epitaxial lift-off method requires a technique for peeling a solar cell film having a large area for a short time, since it is more efficient that a solar cell film having a large area is joined onto the sacrifice substrate at one time than the case where solar cell films each having a small area are joined onto the substrate many times. The shorter the period during the sacrifice layer is etched becomes, the shorter the period required for fabricating the solar cell becomes.
Non Patent Literature 2 (J. J. Schermer et al., Applied Physics Letters 76(2000)2131-2133) and Non Patent Literature 3 (J. J. Schermer et al., Physica Status Solidi 202(2005)501-508) disclose a method for peeling a GaAs solar cell film having a large area from a GaAs substrate in a short time.
FIG. 11(a) shows the method disclosed in Non Patent Literature 2. A sacrifice substrate 701 including a sacrifice layer 702 and a solar cell film 703 is adhered to a sheet for joining 704 (hereinafter referred to as “for-joining-sheet”) by using an adhesive such as wax. A weight 707 is attached to the edge of the for-joining-sheet 704, and the sacrifice layer 702 is subjected to a hydrofluoric acid solution 705 to etch the sacrifice layer 702. The gravity 708 of the weight 707 draws downwardly the for-joining-sheet 704. The solar cell film 703 which is being peeled from the substrate 701 is bent downwardly, and thus the sacrifice layer 702 is more subjected to the hydrofluoric acid solution 705. As a result, the solar cell film 703 is peeled from the substrate 701.
The period required for peeling the solar cell film 703 from the sacrifice substrate 701 is shorter than the period required in a case where the weight 707 is not used. Accordingly, the solar cell film 703 with a large area can be peeled from the substrate 701 in a short time.
FIG. 11(b) shows the method disclosed in Non Patent Literature 3. Instead of withdrawing the for-joining-sheet 704 by the weight 707, in FIG. 11(b), a for-joining-sheet 804 is rolled by a cylinder 806.
In particular, the substrate 801 including a sacrifice layer 802 and a solar cell film 803 is adhered to a for-joining-sheet 804 by using an adhesive such as wax. Next, the edge 809 of the for-joining-sheet 804 is fixed to the cylinder 806. The sacrifice layer 802 is subjected to a hydrofluoric acid solution 805, and the sacrifice layer 802 is etched to peel the solar cell film 803 from the substrate 801. While the cylinder 806 is rotated clockwise, the substrate 801 is move to the left to peel the solar cell film 803 from the substrate 801.